Different purity degrees of low-melting metals are required for using thereof in various branches of industry: from technical purity grade with a content of the main substance 99.9-99.99 mass % to special purity grade with a content of the main substance 99.999-99.99999 mass %, the control being performed with respect to a rather wide range of impurities.
To purify these metals, a set of various processes is usually used such as electrochemical refining, distillation, crystallization and electrophysical purification. These processes, as a rule, make it possible to remove impurities with specific properties, for instance, less or more volatile, with a separation coefficient much more or less than unity, or characterized by a different location in the electromotive series. In addition, the use of the above-cited processes in a number of cases calls for a preliminary, sufficiently complete, purification of the metal from impurities.
At the stage of preliminary purification of gallium and mercury use is made of such procedures as filtration and hydrochemical treatment in solutions of various compounds such as acids, alkalis, salts.
One of the main methods of purification of low-melting metals, obtained by extraction thereof from different raw materials, is treatment of the melt with aqueous solutions of acids or alkalis or treatment of the melt with an aqueous solution of acids and then with an aqueous solution of alkalis, or vice versa.
Thus, the process described in British patent application No. 1,317,478, cl. C7B, 1973, resides in that liquid gallium and mercury are passed dropwise through an aqueous solution of sodium hydroxide and hydrochloric acid, respectively. The impurities which have a higher redox potential pass into the aqueous solution and the metal becomes purified. The disadvantage of the process consists in considerable losses of the metal being purified due to dispersion thereof and entrainment of the smallest drops of the molten low-melting metal with the solution, as well as due to chemical dissolution. Losses of metallic gallium amount to 10% of the initial gallium content.
According to another known process disclosed in Federal Republic of Germany Offenlegungsschrift Application No. 2,304,063, cl. 40a 43/00, 1974, liquid low-melting metal containing impurities, for instance, mercury, is subsequently treated, under intensive stirring with a solution containing mercury sulphate and sulphuric acid at a concentration of 0.05-5 M at 10.degree.-120.degree. C., after which electrolysis is performed; as a result the content of impurity (iron) is reduced to 5.10.sup.-6 in mass. The disadvantages inherent in this process are similar to those inherent in the above-cited process.
In addition, during the electrolysis an amalgam saturated with impurities is formed; said amalgam must be treated and subjected to mercury extraction.
The main and common disadvantage of the known processes involving hydrochemical treatment of low-melting metals with solutions of acids, alkalis, or salts, resides in that upon such treatment only the impurities are removed which are less noble than the metal being purified. Besides, the known processes are performed at 60.degree.-90.degree. C., which results in a considerable chemical dissolution of the metal being purified.